1. Field of the Invention
The present invention relates in general to a heat exchanger for an air conditioner and, more particularly, to a heat exchanger having a slit type grille in each flat fin and making the air currents passing by the fin, such as room air currents, become turbulent flows and mixing the above air currents together, thereby improving the heat exchanging efficiency and reducing the cavitation zone formed in the back of each heat transfer pipe.
2. Description of the Prior Art
With reference to FIG. 1, there is shown the construction of a conventional heat exchanger for an air conditioner. As shown in the above drawing, the conventional heat exchanger includes a plurality of regularly spaced flat fins 1. The fins 1 are vertically arranged such that they parallel each other. A plurality of heat transfer pipes 2 are angularly fitted into the fins 1 such that the pipes 2 are perpendicular to the fins 1. The air currents flow in the space defined between the fins 1 in the direction of the arrow in FIG. 1 and exchange heat with the fluid flowing in the heat transfer pipes 2.
A thermal fluid flowing about each flat fin 1 has the characteristic in that the thickness of the thermal boundary layer 3 on both heat transfer surfaces of the fin 1 is gradually increased in proportion to the square root of the distance from the air current inlet end of the fin 1 as shown in FIG. 2. In this regard, the heat transfer rate of the fin 1 is remarkably reduced in proportion to the distance from the air current inlet end. Therefore, the above heat exchanger has a lower heat transfer efficiency.
When lower velocity air currents flow in the direction of the arrow of FIG. 3, the thermal fluid flowing about each heat transfer pipe 2 has the characteristic in that the air currents separate from the outer surface of the pipe 2 at portions spaced apart from the stagnation point of the pipe 2 at points spaced apart by an angle 2.theta. in the range of 70.degree..about.80.degree. . Therefore, a cavitation zone 4 is formed in the back of the pipe 2 as shown in the cross-hatched region of FIG. 3. In the above cavitation zone 4, the heat transfer rate of the pipe 2 is remarkably reduced so that the heat transfer efficiency of the above heat exchanger become worse.
In order to overcome the above problems, Japanese U.N. Laid-open Publication No. Sho.55-110995 proposes an improved heat exchanger for air conditioners. As shown in FIG. 4, the above Japanese heat exchanger includes a plurality of heat transfer pipes 2 which are fitted into the regularly spaced flat fins 1 such that the pipes 2 are perpendicular to the fins 1. The above heat exchanger also includes a plurality of slit type grille which are formed beside the pipes 2 on each fin 1. Each slit type grille is formed by vertically slitting a given portion of the fin 1 several times and alternately bending the remaining strips in opposite directions, thereby forming a plurality bent strips 5a, 5b, 5c, 5d, 5e and 5f in the fin 1.
In other words, three strips 5a, 5c and 5e are bent to one side of the fin 1 such that the strips 5a, 5b and 5c are regularly spaced apart from each other. However, the other three strips 5b, 5d and 5f placed between the above strips 5a, 5c and 5e are bent to the other side of the fin 1.
The above heat exchanger having the plurality of slit type grilles on each flat fin 1 causes the heat exchanging fluid to become turbulent due to the above grilles, thereby reducing the thickness of the thermal boundary layers formed on the fins 1. As the above heat exchanger has thin thermal boundary layers formed on the fins 1 due to the slit type grilles, this heat exchanger somewhat improves the heat transfer efficiency in comparison with the conventional heat exchanger having the flat fins 1 with no slit type grilles. When the partial heat transfer capacities of the heat exchanger are measured, the upstream strips 5a and 5b create the thin thermal boundary layers, thus to improve the heat transfer efficiency. However, as the downstream strips 5c to 5f are included in the thermal boundary layers formed by the upstream strips 5a and 5b, the downstream strips 5c to 5f can not improve the heat transfer efficiency. In addition, the cavitation zone is still formed in the back of each heat transfer pipe 2. Furthermore, the air currents flowing in the space defined between the flat fins 1 are not mixed together but become laminar. Therefore, the above Japanese heat exchanger is not expected to improve the heat transfer efficiency which would be improved if the air currents are mixed together.